HtmlAgilityPack中的NullReferenceException

Question

HtmlAgilityPack中的NullReferenceException

c# html-agility-pack

题

我试图从下面提到的URL使用xpath提取link

string url = "http://www.album-cover-art.org/search.php?q=Ruin+-+Live+Album+Version+Lamb+of+God"

我的代码：

HtmlAgilityPack.HtmlWeb web = new HtmlAgilityPack.HtmlWeb();
HtmlAgilityPack.HtmlDocument htmlDoc = new HtmlAgilityPack.HtmlDocument();
htmlDoc = web.Load(url); //Exception generated here Line 23

if (htmlDoc.DocumentNode != null)
{
  HtmlNode linkNode = htmlDoc.DocumentNode.SelectSingleNode(".//*[@id='related_search_row']/img/@src");
  if (linkNode != null)
        Console.WriteLine(linkNode.InnerText);
}

上面的代码编译得很好，但是当我尝试运行它时会产生异常

Unhandled Exception: System.NullReferenceException: Object reference not set to an instance of an object.

完成堆栈跟踪

System.NullReferenceException: Object reference not set to an instance of an object.
   at HtmlAgilityPack.HtmlDocument.ReadDocumentEncoding(HtmlNode node) in C:\Source\htmlagilitypack\Trunk\HtmlAgilityPack\HtmlDocument.cs:line 1916
   at HtmlAgilityPack.HtmlDocument.PushNodeEnd(Int32 index, Boolean close) in C:\Source\htmlagilitypack\Trunk\HtmlAgilityPack\HtmlDocument.cs:line 1805
   at HtmlAgilityPack.HtmlDocument.Parse() in C:\Source\htmlagilitypack\Trunk\HtmlAgilityPack\HtmlDocument.cs:line 1468
   at HtmlAgilityPack.HtmlDocument.Load(TextReader reader) in C:\Source\htmlagilitypack\Trunk\HtmlAgilityPack\HtmlDocument.cs:line 769
   at HtmlAgilityPack.HtmlWeb.Get(Uri uri, String method, String path, HtmlDocument doc, IWebProxy proxy, ICredentials creds) in C:\Source\htmlagilitypack\Trunk\HtmlAgilityPack\HtmlWeb.cs:line 1515
   at HtmlAgilityPack.HtmlWeb.LoadUrl(Uri uri, String method, WebProxy proxy, NetworkCredential creds) in C:\Source\htmlagilitypack\Trunk\HtmlAgilityPack\HtmlWeb.cs:line 1563
   at HtmlAgilityPack.HtmlWeb.Load(String url, String method) in C:\Source\htmlagilitypack\Trunk\HtmlAgilityPack\HtmlWeb.cs:line 1149
   at HtmlAgilityPack.HtmlWeb.Load(String url) in C:\Source\htmlagilitypack\Trunk\HtmlAgilityPack\HtmlWeb.cs:line 1107
   at ScreenScrapping.Program.Main(String[] args) in c:\Users\ranveer\csharp\ScreenScrapping\ScreenScrapping\Program.cs:line 23

所以，我的问题是为什么我得到这个例外。

RanRag

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Alex · Accepted Answer · 5/23/2017 12:31:00 PM

这是HtmlAgilityPack中的一个错误。您尝试解析的文档有<meta http-equiv="Content-Type" content="text/html; charset=iso-utf-8">其中charset值（ iso-utf-8 ）不能被AgilityPack解析为有效的编码名称。正如Simon Mourier 所说， 这是1.4.0.0中引入的错误 。

要避免这种情况，请从流中手动加载文档并手动设置编码，如下所示：

var htmlDoc = new HtmlDocument();
htmlDoc.OptionReadEncoding = false;
var request = (HttpWebRequest)WebRequest.Create(url);
request.Method = "GET";
using (var response = (HttpWebResponse)request.GetResponse())
{
    using (var stream = response.GetResponseStream())
    {
        htmlDoc.Load(stream, Encoding.UTF8);
    }
}

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